400 Conduction in Solutions and Gases, 



by showing that when a sheet of metal is negatively electrified 

 and exposed to ultra-violet light, the adjacent air is thrown 

 into a state which permits the charge to leak rapidly away. 



Interest was now thoroughly aroused in the problem of 

 conductivity in gases ; and it was generally felt that the best 

 hope of divining the nature of the process lay in studying the 

 discharge at high rarefactions. " If a first step towards under- 

 standing the relations between aether and ponderable matter is 

 to be made," said Lord Kelvin in 1893,* " it seems to me that 

 the most hopeful foundation for it is knowledge derived from 

 experiments on electricity in high vacuum." 



Within the two following years considerable progress was 

 effected in this direction. J. J. Thomson,^ by a rotating-mirror 

 method, succeeded in measuring the velocity of the cathode rays, 

 finding it to be| 1*9 x 10 7 cm./sec. ; a value so much smaller than 

 that of the velocity of light that it was scarcely possible to 

 conceive of the rays as vibrations of the aether. A further 

 blow was dealt at the latter hypothesis when Jean Perrin, 

 having received the rays in a metallic cylinder, found that 

 the cylinder became charged with resinous electricity. When 

 the rays were deviated by a magnet in such a way that they 

 could no longer enter the cylinder, it no longer acquired a 

 charge. This appeared to demonstrate that the rays transport 

 negative electricity. 



With cathode rays is closely connected another type 

 of radiation, which was discovered in December, 1895, by 

 W. C. K6ntgen.ll The discovery seems to have originated 

 in an accident : a photographic plate which, protected in the 

 usual way, had been kept in a room in which vacuum-tube 

 experiments were carried on, was found on development to show 

 distinct markings. Experiments suggested by this showed 



* Proc. Roy. Soc. liv (1893), p. 389. 

 t Phil. Mag. xxxviii (1894), p. 358. 



The value found by the same investigator in 1897 was much larger than this. 

 $ Cornptes Rendus, cxxi (1895), p. 1130. 



|| Sitzungsber. der Wiirzburger Physikal. -Medic. Gesellschaft, 1895 ; reprinted, 

 Ann. d. Phys. Ixiv (1898), pp. 1, 12; translated, Nature, liii (1896), p. 274. 



